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The Stone Corner

Stone Corner - Pipestone Nov/Dec 2003

 

Q: What is pipestone and does it come only from Minnesota?


A: “Pipestone” is a much generalized term meaning exactly what it looks like, stone used to make smoking pipes. Throughout the millennia and among numerous cultures worldwide, many types of stone have been carved into pipes, probably at least as many as the kinds of things smoked in them. The requirements seem to be ease of working and some inherent esthetic appeal, often color.


Commonly carved materials include 1. soapstone (talc, steatite), 2. chlorite, 3. pyrophyllite, 4. specific clay minerals such as kaolin or sepiolite (meerschaum), and 5. argillite, deposits of very fine-grained mixtures of various minerals hardened to varying degrees by metamorphism, heat, and pressure. Argillites have many different colors and all can occur as massive deposits showing little or no bedding or layering so they carve uniformly in all directions. Material found in the Queen Charlotte Islands and carved by Haida tribal members is a well-known black argillite. Terminology can get a bit fuzzy here because we have geologists, archeologists, ceramicists, museum curators, art dealers, and stone dealers all using these same names but not always with the same meanings.


In North America “pipestone” is often taken to mean red material, previously (but erroneously) thought to come only from a small area in southwestern Minnesota. Red pipestone artifacts have been found at archeological sites throughout the continent and the material is still of unique importance in many Native American cultures. The Minnesota pipestone is also called catlinite. The entire catlinite deposit lies within the Pipestone National Monument and can be quarried only by Native American tribal members. It is not the only occurrence of red pipestone but properly it is the only catlinite.


Continental glaciers carried red argillite from other occurrences and deposited it throughout the upper Midwest and many of the original source locations have been found. Detailed scientific studies have shown that most red pipestones contain varying proportions of kaolin, pyrophyllite, diaspore, muscovite, and quartz. The red color is from hematite, iron oxide, which coats the mineral grains and fills the spaces between them. Some deposits, including catlinite, have their own unique proportions of these minerals — a chemical and mineralogical fingerprint. Catlinite, for example, contains pyrophyllite, diaspore, muscovite, and a little kaolin, but no quartz. With appropriate analysis it is possible to determine if a sample or artifact is catlinite from the Minnesota locality or red argillite from some other area.

 

Stone Corner - Travertine July/Aug 2003

 

Q: What causes the holes in travertine and do they affect carving?


A: The kind of travertine with holes in it is a form of limestone, calcium carbonate, or calcite, deposited from hot springs and hot lakes. When thermal water saturated with carbon dioxide and calcium reaches the earth’s surface, some of the carbon dioxide bubbles off or is removed by biologic activity. This changes the water chemistry causing calcite to crystallize as thin sedimentary layers and as coatings on the cell walls of bacteria and algae and on the stems and leaves of more complex plants growing in the water.


Eventually the organisms die and decay leaving cavities within and between cells and plant fragments. The cavities range from microscopic to an inch or more. After initial deposition continued circulation of saturated water may add more calcium carbonate, filling many of the holes. Evidence of infilling can often be seen on polished surfaces as a rim or layer lining the larger cavities. The smaller ones are already filled. This lack of porosity, in spite of appearances, is why travertine is useful as building stone.


Travertine has been quarried in the Siena and Tivoli areas of Italy for over 2000 years and material from those quarries is still exported around the world. Turkey, Spain, and Mexico are also major modern producers and many countries export smaller amounts. Although there are many occurrences in the United States, only three are currently in production: southwestern Montana near Yellowstone Park, southeastern Idaho, and central New Mexico south of Albuquerque.


Carving travertine does require some extra attention. A cluster of holes can behave as if the stone were softer and it’s easy to over-carve or over-grind. Carving from more dense stone up to a hole can fracture the edges deeper than you intended, especially if you are using a point. Your choice then is to leave the tool mark or more carefully carve the entire surface deeper to remove the mark. As I approach the final surface I use a wide claw, an inch or more with 5 or 6 teeth, which spans the entire hole reducing the risk of over-breaking. Grinding or sanding with a rigid disk is another solution. Using your fingers or some other flexible backing in the final stages of sanding can round the lip of a cavity creating a surface that reflects a narrow rim of light around each hole. You may or may not want that.


I made the qualification in the first sentence because dense, banded, translucent limestone deposited from cold springs and as various cave formations is also called travertine. Other names used include onyx marble, Mexican onyx, and (incorrectly) alabaster. To add to the confusion the term “onyx” with no modifiers means a banded form of quartz, which is considerably harder than calcite.

 

Stone Corner - Anhydrite May/June 2003

 

Anhydrite has not been a widely popular stone, but its availability on the west coast of North America and the beauty of its finish are likely to result in more sittings in sculpture exhibits in the future. It has the pearly luster of alabaster and yet carves and finishes more like harder stone.


Geology

Anhydrite is both a mineral and a sedimentary rock. Its name comes from the Greek for “without water:” essentially it is the anhydrous form of calcium sulfate (CaSO4). Anhydrite is the sister or brother of alabaster (gypsum), which is the hydrated form of calcium sulfate (CaSO4: 2H2O). Two molecules of water are attached to each molecule of calcium sulfate in gypsum.


Anhydrite commonly is found in white, gray, brown, and light red, or pink. It is heavier than alabaster, weighing in at about 187 pounds per cubic foot with a specific gravity of 3.0. One of the key differences between alabaster and anhydrite is the hardness: alabaster is 2 and anhydrite ranges from 3 to 3.5. It is insoluble in hydrochloric acid and sulfuric acid; however, it is easily attacked by water. In the earth, anhydrite converts to gypsum when it absorbs water, and in your yard it will break down with facility, if exposed to the elements.


Anhydrite is one of a group of rock-forming minerals called evaporites, so called because they form by the precipitation of the minerals from evaporating brines or salt water. The other evaporites are gypsum and halite (salt). They form in shallow salt water seas that are alternately submerged and dewatered. Beds of the rock can be found in thicknesses of a few feet to hundreds of feet. Which mineral is precipitated at any time depends on the temperature, pressure, and salinity of the water. At higher temperatures, anhydrite is the first one to precipitate, followed by gypsum; however, at lower temperatures, the opposite is the case. Halite is normally the last to form. In the laboratory, and presumably in nature, gypsum and anhydrite can be made to convert to one another by the changing of pressure and temperature. It is also known that anhydrite can be changed to gypsum by the addition of water, such as during the weathering process. Conversely, the reverse can be achieved by the baking or drying of gypsum.


In North America, anhydrite is found in the salt domes of Louisiana and Texas and in stratified deposits in Nova Scotia, New York, New Mexico, and British Columbia. It is also in large layered salt deposits of Poland, Germany, Austria, France, and India. Anhydrite is not considered an important industrial mineral and is treated as a contaminant in large deposits of gypsum. It is used as a source of sulfuric acid, as a retardant for concrete curing, for a filler in paper, and as a soil conditioner.


Quarrying

Although anhydrite is relatively hard stone, it is somewhat easy to mine because it is naturally broken by numerous joints. This makes mining anhydrite, for sculpture, easily done without blasting, which would shatter and make unusable much of the stone. It is removed from the ground or rock face by hand tools or with a backhoe. The best method for determining the integrity of the stone is to tap the stone with a hammer and listen carefully for changes in the ring.


Working Anhydrite

Anhydrite is brittle and hand tool working is not recommended. Although a hardness of 3 to 3.5 is not extreme, sculptors who have worked the stone definitely prefer power tools. Hand chisels tend to create small chips in the surface, and sharp corners are commonly lost when unexpected breaks occur. The good part is that anhydrite is unlikely to bruise, in the manner of alabaster. It can be worked with a hand-held grinder as well as a stationary wheel, and the stone cuts very evenly and easily with such tools. Anhydrite takes well to a grinder and carbide disks will show little wear after working the stone.


Anhydrite can be taken to a 70 to 600 grit finish, depending on the color and pattern of the stone, and buffed with tin oxide using a wet rag or a mechanical buffer. Of all the colors, the translucent white produces the most lustrous finish although, the brown may have the most interesting patterns.


Flaws

Cracks and joints are not common flaws in anhydrite. In general, it is a solid stone. The white variety tends to be the most consistent in color and hardness, containing scattered soft spots. Sometimes, small white spots may be present in the stone.


Although not exactly a flaw of the stone, a major weakness of all anhydrite is its susceptibility to water. It must be kept indoors before and after carving. If left outdoors, it will absorb water and deteriorate.


Safety

No particular safety hazards are reported for anhydrite however, we all need to remember to protect our lungs, eyes, and ears at all times. Be a sage and safe sculptor.


Appreciations

Thanks to Randy Zieber of Vancouver, British Columbia for his quarrying and carving expertise and to Carol Way of Seattle, Washington and Vic Picou of Burbank, California for sharing their carving experiences with me.

 

Stone Corner - Jade Jan/Feb 2003

“The Stone of Heaven,” is one of the most revered natural substances in the world, such has been the case for centuries, particularly in Asia. What other mineral would make a Chinese emperor offer fifteen cities for a jade carving that he could hold in the palm of his hand or make Montezuma smile when he heard that the Spaniard Cortez was interested only in gold, since Montezuma’s most precious possession was jade.

Since Neolithic times, no other mineral has been so venerated, nor so often intertwined with the dead to accompany them into the afterlife. In this life, to the Chinese, jade embodies the five cardinal virtues of life: charity, modesty, courage, justice, and wisdom. But China and Asia have not had a monopoly on jade; jade art and tools have been found in the Maori, Olmec, Aztec, and Mayan cultures, and among the NW American Coast Indians and Eskimos.


The English word jade has a circuitous derivation. It started with the Spanish expression “piedra de hijada”, meaning the “stone of the loins”, because it was claimed that this stone could cure diseases of the kidneys. This gave rise to the word nephrite, from the Greek word for kidneys: nephros. The French equivalent l’ejade eventually evolved into le jade, and its English translation, jade.


Physical Traits

Jade is actually two minerals: nephrite and jadeite. Nephrite is the ancient stone, a calcium magnesium silicate: Ca2 Mg5 Si8 O22 (OH)2. Jadeite is the upstart of the two, having been discovered only 200 years ago, it is a sodium aluminum silicate: Na A1 Si2 O8. Both are insoluble in acids.


Nephrite is a tough, compact variety of tremolite and actinolite (amphiboles) with a specific gravity of 3.0 to 3.3, and a hardness ranging from 5 to 6 on the Mohs Hardness Scale. It is considered to be the world’s toughest stone. Although it is not as hard as jadeite, it is much harder to break than its cousin. The reason for this is the filament-like crystalline structure of the mineral.  When polished, nephrite has a soapy or oily luster. It is commonly referred to as “mutton-fat” jade, because of the marbled appearance that resembles animal fat. Nephrite is normally associated with serpentine, hornblende gneiss, and schist.


Jadeite is a hard, brittle mineral with a 3.3 to 3.5 specific gravity and a hardness of 6.5 to 7. It is pyroxenes, whose crystals are shorter and granular, and are more closely interlocked, as in a mosaic. Although it is harder than nephrite it fractures more easily. When polished, jadeite is vitreous or glassy. It is commonly found in association with albite feldspar, nepheline, and quartzite and is surrounded by serpentine.


Pure jade is white or transparent; the transparent variety is very rare. All other colorations of the stone owe to the addition of other elements. Although the five traditional colors of Chinese jade are red, black, white, yellow and green, jade can be found in all colors of the rainbow. Both nephrite and jadeite may be white or transparent or other colors. However, the vitreous apple and emerald green versions are almost assuredly jadeite. The milky, silky, or “mutton-fat” appearance is a clue that nephrite is the mineral inside. In general, the bold colors are never seen in nephrite, they are reserved for jadeite.


Geology

Jadeite and nephrite are minerals formed in a metamorphic environment associated with high pressure and a low temperature. While there are as many as seven hypotheses to explain the formation of the jade minerals, the most plausible is that they form under anomalously high water pressures on the lower plate of a low-angle thrust fault. In these circumstances, the fluid pressures may exceed the pressure of the rock overburden.


Such conditions may occur at the margins of the earth’s plates, such as in the Alps, California, the northwest North American coast, and the Asian part of the Pacific Rim.


For twenty centuries, the main source of jade was the nephrite from China, found primarily as cobbles and boulders in the rivers and creeks. The river stones have a tough oxidized rind, making them not easily distinguishable from other stones. The Khotan and Yarkand regions, once the prime providers, are reportedly mined out of nephrite now, and nephrite boulders are only occasionally encountered. In 1784, jadeite was discovered in northern Burma. Originally, it was also in the form of river cobbles and boulders. However, eventually the mother lode was found and mined. Although Burmese jade is not a large part of the international jade market because of political conditions there, these mines are still in operation. Jadeite was also located in China’s Yunnan Province.  Pre-Columbian implements and works of art in jadeite are thought to have originated in Guatemala.


Around the world, other sources of nephrite are: Lake Baikal, Russia; New Zealand; Switzerland; Zaire; Jordanow Slaski, Poland; British Columbia, Canada; Kotzebue, Alaska; Lander, Wyoming; and San Benito, California. Northern British Columbia now has the world’s largest active nephrite mine near Dease Lake. About 300 tons are exported to Asia annually, and about 7 tons are used in North America for carvings and jewelry. Jadeite comes from Japan, western California, the Celebes, New Guinea, and Guatemala.


Jade does not have any other use in the modern world, except as a precious mineral and carving medium. In ancient times, it was also used to make household implements and weapons.


Quarrying

In the old days in Asia, jade was mined by building a charcoal fire near the mineral vein.   At night, when the temperature dropped, the stone in the vein would crack. Workers would then drive stakes into the cracks to keep it open. Holes were drilled into the cracks, which were then filled with water so the freezing water would further expand the crack, eventually breaking a block free.


Modern methods are not entirely different. Drilling and blasting, along with heavy machinery, are used to remove the overburden of soil and rock. In British Columbia, large blocks and slabs are then removed from the mountain by drilling (2-inch-diam. holes) and hydraulic splitters. Hydraulic splitters are the mechanized versions of feathers and wedges. The low-quality nephrite is then separated from the high-quality with a 72-inch diameter saw. In general, the larger blocks are 5 to 10 tons, but the largest for a special carving was 25 tons. Cobbles and boulders are still used when found, but they are a minute proportion of the yield compared to years past


Working Jade

Jade is too hard to carve with conventional stone carving tools such as hammer and chisels. It is worked by grinding or abrading the stone away. From ancient times to only a few decades ago, jade was shaped by using hand-dipped quartz abrasives, along with hand tools, foot treadle machines, and bow drills. Presently, cutting and shaping of the stone is accomplished with diamond saws, drills, and grinding wheels.


Although tungsten carbide tools can be used with jade, the most cost effective are diamond tools. Diamonds can be used on all of the tools described, including the tiny bits that fit on the Foredom machine. The diamond sintered points with seven different grits can get into the smallest nooks and crannies. Sanding is normally accomplished with diamond compound that can be obtained in a lapidary shop. Deborah Wilson’s secret polishing compound recipe contains diamond powder, vaseline, and lipstick (for tracking where you have polished). A water bath is used during grinding and polishing. The diamond polishing compound however, is only used dry.Flaws


When selecting jade, it is best to have two sawn sides to view. Early October is the best time of the year to obtain good pieces of the stone, because the best pieces of jade are still available soon after the summer quarrying season. White streaks in the otherwise green stone can be softer zones that may not work consistently with the rest of the stone. Some of the inclusions that may be harder than the rest of the stone are blue-gray streaks, garnets, and flecks of chromite. Chromite inclusions in the finished piece have appeal to North Americans, but not the Chinese, who prefer pure colors.


Safety

Because jade contains silica in both minerals and nephrite is comprised of actinolite and tremolite, it is important to maintain a constant stream of water on the stone during grinding and to wear a respirator with very fine dust filters. Don’t forget about eye protection.


Appreciations

Thanks to artist, Deborah Wilson of Vernon, British Columbia for sharing her jade carving experiences and to quarry operator and jade merchant, Kirk Makepeace of Jade West in White Rock, British Columbia for information on quarrying and availability of jade.

 

Stone Corner - Sandstone - July/Aug 2002

Sandstone has been a reliable utility stone throughout the centuries.  Many cities, both ancient and modern, take advantage of its fine qualities.  It is easy to quarry, found throughout the world, is relatively easy to shape and carve, and resists erosion in most climates.  While it is limited in colors, it is commonly uniform, compared to other stones, and therefore provides the architect with a useful building stone.

Geology

In very basic terms, sandstone is a sedimentary rock that is nothing more than cemented sand grains.  If grain size gets too large, the rock is a conglomerate; if the grains are too fine, the rock is a claystone, shale, slate or argillite.  There are several types of sandstone: arkose, arenite, graywacke, orthoquartzite and protoquartzite.  The classification depends on the kind of cementing agent and the percentage of quartz and feldspar sand particles. The chief types of cementation include silica (quartz. opal and chalcedony), calcite, dolomite, clay and limonite.  The cementing agent can either be deposited at the same time as the sand particles or at a later date.

The grains themselves can be derived from other existing rocks that are nearby, and they may be any shape, although they are commonly rounded to subrounded owing to collisions with other particles during transportation.  They may have accumulated in ancient sand dunes, on river bottoms, in the shallow portion of deltas of fresh water lakes or in a shallow marine environment.  Very slowly over millions of years, the deposits of sand are compacted by overlying rock strata.  The pressure and the cement cause the rock to gain strength.  As a general rule, the older the rock the higher the strength.


Because sandstone is deposited in environments where organisms live, fossils are common.  For similar reasons, concretions - hard nodes with interesting shapes - are also found in sandstone.  The concretions themselves are sometimes mined and their interesting shapes are objets d’art. Because sandstone is deposited in environments where organisms live, fossils are common.  For similar reasons, concretions - hard nodes with interesting shapes - are also found in sandstone.  The concretions themselves are sometimes mined and their interesting shapes are objets d’art.

As a sedimentary rock, it is deposited in layers, and is commonly interbedded with shale, limestone and coal.  Where the four repeat in sequences, they are called cyclothems. Where higher depositional energy is involved, the gravel or cobbles change the rock designation to conglomerate.

As a rock (as compared to a mineral), sandstone does not have a designated hardness. Its hardness ranges considerably because of the wide range of cementing agents and degrees of weathering.  Caveats aside, sandstone probably ranges from 3 to 6 in hardness.


There are, of course, softer sandstones; however, they would probably not hold any sculptural detail and would spall badly in the elements.  The hardness of sandstone is normally expressed in terms of its unconfined compressive strength, the strength obtained by compressing a cylindrical piece of the stone to its breaking point.  Unconfined compressive strength of stone suitable for buildings and sculpture is between 5,000 and 30,000 pounds per square inch (psi).  Wilkeson sandstone is reported to be one of the  strongest sandstones in the world.  Sandstone weighs between 136 and 166 pounds per cubic foot.

Sandstone is most commonly brown or gray, however, white, yellow, green and red exist.  Some sandstone has layers or streaks of red iron oxide, which may make it attractive for sculpture, but not desirable as a building stone.


Its most common use worldwide is in buildings. Can you guess what rock type are the “brownstone” buildings of eastern U.S. cities?  Sandstone is also used for curbstones, bridge abutments and retaining walls, because it can be quarried relatively easily to very close tolerances.  It can be pulverized into sand particles to use as sandblasting material or foundry sand.  The hardest sandstones have been used as grindstones and sharpening stones.


Quarrying

Sandstone is extracted in large open pit surface mines, generally from large faces of exposed rock.  Because of its stratification, it naturally divides in one dimension.  The  somewhat horizontal bedding provides a convenient plane on which the rock breaks.  Planes of weakness in the other two planes commonly develop during compression or tension of the earth’s crust.   However, these planes of weakness may not be the correct size for building or sculpture stone.  Therefore, cutting, drilling and light blasting are sometimes required to remove the stone.


In Washington State, Wilkeson, Tenino and Chuckanut sandstones have all been removed by cutting channels or slots in the rock and then drilling a row of holes along the bottom of the channeled rock.  A row of drill holes across the back of the channeled section also separates the stone from the mountain.  The width of the channels can be varied to render different thickness of slabs or blocks.  Feathers and wedges can be used to bring the stones closer to the desired size.  The need for carbide or diamonds for the drills is governed by the strength of the cementing mineral.


The rectangular blocks of stone are then milled by gang saws and planers to a predetermined size that closely fits the final carved stone.


Working Sandstone

Sandstone is chosen commonly for its uniformity of color and grain size.  A uniform block is essential for a good sculptural stone.  Some of the most common flaws in sandstone that are unique to that stone are coal seams, fossils and concretions.  In addition to being unsightly, the coal seams are weak and can cause the piece to split. Fossils can either be harder than the stone or much softer; if softer, they can fall or spall out leaving a hole in the finished piece.  Concretions are hard nodes within a softer rock that form around a small nuclear particle and are bonded with a harder cement, commonly iron oxide.  As with many rocks, small seemingly undetected fractures in the stone can result in the splitting of the stone after it has been worked for some time.


Sandstone can be roughed out with a diamond or carbide skill saw.  A point can be used to take the shape within about 1/2-inch of the final shape.  Shaping can be accomplished with a toothed chisel, flat chisel and a cape chisel, a narrower version of a flat chisel. A “bull-nose” chisel is used to create concave surfaces  Sandstone carvers have traditionally used wooden mallets instead of iron or steel.  The wooden mallets are strong enough to work sandstone and yet they reduce the noise and seem to absorb much of the energy before it reaches the hand and arm.


Smoothing of sandstone can be performed with different implements, depending on the desired final effect.  Initially, a carbide scraper or rasp is used, and then final polishing can be done with another piece of sandstone or with a diamond file. The latter is definitely recommended when working with Wilkeson sandstone.

Sandstone can be either an indoor or outdoor stone, the main differentiating factor being porosity.  If the stone has a high porosity, such that water can fill the voids,   it is likely better indoors.  Left outdoors, moisture and freeze-thaw would soon soften the surface of the sandstone and ruin any details.  Look at a piece of sandstone on a building sometime and note the occasional scallop out of the stone facade.  Many of the older building of Europe require restoration after a century or two.  The most effective deterrent is periodic cleaning of the stone.


And remember !!   Keep your goggles and mask on.  The dust is very fine grained and more than likely contains silica.  Silicosis can be debilitating and even fatal.

Appreciations


Thanks to stone carver Keith Phillips of Tenino, WA, for sharing his knowledge and expertise of sandstone with us. Keith’s enthusiasm for the history of quarrying and carving of sandstone are an inspiration to all who meet him